This invention relates to a fluid flow rate control system for controlling the flow rate of a fluid (air or fuel) being supplied to an internal combustion engine, to thereby regulate the air/fuel ratio of a mixture composed of air and fuel, and more particularly to a device provided in such a control system and operable at the start of the engine to set a pulse motor used as an actuator for driving a fluid flow control valve at a predetermined initial position.
An air/fuel ratio control system is already known which is provided in combination with an internal combustion engine provided with a three-way catalyst mounted at the exhaust gas outlet and is operable in response to a signal from an exhaust gas ingredient sensor mounted at the exhaust gas outlet upstream of the three-way catalyst to achieve feedback control of the air/fuel ratio of an air/fuel mixture being supplied to the engine. An arrangement is also known that an air/fuel ratio control valve i.e., a fluid flow rate control valve is provided in an air passage connected to the air bleed or fuel passage of the carburetor or in an air passage directly connected to the intake manifold, the valve being arranged to be driven by a pulse motor.
However, the control circuit provided in such a conventional system using a pulse motor for driving the air/fuel ratio control valve is not adapted to determine the actual position of the air/fuel ratio control valve at the start of the engine. Therefore, conventionally air/fuel ratio control is started with the position of the air/fuel ratio control valve held at an initial position which the valve assumed immediately before the start of the engine. With such initial valve position, it is difficult to ensure good startability and driveability of the engine and low exhaust emission characteristics at the start and warming-up of the engine. Particularly, immediately after the start of the engine, usually the feedback control circuit is brought into an open loop mode, since the exhaust gas ingredient sensor which is usually formed of an O.sub.2 sensor is still inactive just after the start of the engine. On this occasion, if no control signal is given to the air/fuel ratio control valve for moving it from its initial position to a proper position, it will be the more unlikely that the requirements for the above-mentioned engine performances are fulfilled.
To cope with this situation, one would consider moving the rotor position of the pulse motor to a predetermined position and holding it there by mechanical means at the termination of operation of the engine. However, this method is not preferable, since it would require a structurally complicated device for carrying out this method.
It is preferable that the air/fuel ratio of a mixture being supplied to the engine should have a particular value appropriate for the starting and warming-up operation of the engine in order to obtain required startability, drive ability and exhaust emission characteristics of the engine. To this end, an arrangement that the valve position of the air/fuel ratio control valve is automatically held at a predetermined position at the start of the engine would be advantageous, particularly in a combination of the air/fuel ratio control valve with an air/fuel regulating device such as an automatic choke. Such arrangement facilitates setting of the opening of the automatic choke or the like and enables simplification of a control circuit associated with the air/fuel ratio control valve.